The present invention relates to the art of image processing. It finds particular application in conjunction with black-and-white or monochromatic digital printers, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Mapping full color images to black-and-white allows rendering of the full color image without the associated costs of color processing. That is to say, generally, monochromatic or black-and-white reproduction or rendering is faster and more economical than full color. Additionally, color processing may not always be available. Likewise, it is advantageous to be able to map highlight color images, such as those produced by the Xerox 4850 Highlight Color Printer, to black-and-white or an otherwise monochromatic scheme. A highlight color image generally represents a full color image which has been mapped down to two colors, for example using the technique of commonly assigned U.S. Pat. No. 5,237,517, incorporated herein by reference. Images rendered in a highlight color scheme are typically portrayed by two color separations, a highlight color (usually a primary color) and a base color (usually black or white).
However, it is still important to retain, as much as possible, the information one desires to communicate with the rendered image. For pictorial images, such as photographs, much of the information is in the luminance, and gray scale halftoning techniques work well. However, for presentation graphics, such as charts and graphs, much of the information is found in the hue or color rather than the luminance, and while light colors look light, and dark colors look dark, very different colors (e.g. light blue and green) can have the same luminance level and be indistinguishable when mapped to a gray level. For such images, it is advantageous to map different colors to different black-and-white textured patterns that represent those different hues to preserve the information contained in the different hues used in the image.
The commonly assigned U.S. Pat. No. 5,153,576, incorporated herein by reference, describes a method for automatically generating textured patterns from full color images represented in a red-green-blue (RGB) color space. The full color to texture scheme preserves luminance information and performs the mapping function by dividing a halftone cell into three distinct and separate regions exclusively responsive to corresponding color separations, and allocating the size of the three regions of the halftone cell in proportion to the luminance of the red, green, and blue primary colors. In the RGB color space a white color is produced by full intensity values of all three color separations. With this method, a corresponding white will be printed in response to a white input since each separation will cause its separate region of the cell to print white and therefore the whole cell prints white.
However, the method described in the 5,153,576 patent is not suited to mapping from highlight color schemes to black-and-white textures. The most obvious difference between highlight color and full color is the number of color coordinates. Full color has three and highlight color has two. One approach to handling the difference might be to simply divide the halftone cell into only two regions rather than three. However, this presents a further problem. White and black are both indicated by the presence of no highlight color and all or none of the base color. Therefore, accurate mappings that preserve black and/or white are not achieved when the halftone cell is divided into regions under the exclusive control of one of the color coordinates. For example, with a black base color, black is represented by no highlight and full base color. The output however with the two exclusive regions method would by gray. The region under the control of the highlight color would have zero output because no highlight color was indicated, while the region under the control of the base color would have a full output. The overall output result for the halftone cell is a partial output or gray.
The present invention contemplates a new and improved mapping technique, which overcomes the above-referenced problems and others.